Storage device utilizing e.b.i.c.



Nov. 16, 1965 J. LEMIERT STORAGE DEVICE UTILIZING E B I C Filed Dec. 5, 1962 Fig.l.

Fig.4.

Fig.2.

INVENTOR Joseph Lemperr @AQAflQ ATTbRNE WITNESSES (SW I 4 United States Patent 3,218,504 STORAGE DEVICE UTILIZING E.B.I.C. Joseph Lempert, Pittsburgh, Pa., assignor to Westinghouse Electric Corporation, Pittsburgh, Pa., a corporation of Pennsylvania Filed Dec. 3, 1962, Ser. No. 241,644 2 Claims. (Cl. 315-11) This invention relates to electron discharge devices and more particularly to those having storage target electrodes.

One well known type of storage target electrode tube is the image orthicon. This tube has a target structure usually formed of a thin film of glass. A photocathode and focusing means are provided on one side or write side of the target for directing photoelectrons emitted from photocathode onto one surface of the target to provide a charge pattern thereon corresponding to a radiation pattern focused on the photocathode of the tube. An electron gun, providing an electron beam of low velocity, is positioned on the opposite side or read side of the target with respect to the photocathode. The electron beam scans the read side of the target to read out the information written on the target. One of the basic problems associated with the image orthicon target is that transverse charge neutralization within a frame time of scanning requires that the target be of a relatively low resistivity. 'A target of relatively low resistivity to provide this function results in severely increasing lateral leakage of charges and therefore decreasing storage time.

Another type of well-known pickup tube is that utilizing a target exhibiting the property of electron bombardment induced conductivity. In this type of device, a target is formed of a thin electrically conductive back plate coated with a film of a suitable insulating material such as arsenic trisul-fide on the surface of the back plate remote with respect to the photocathode. The insulating material is of the type which exhibits the property of electron bombardment conductivity. A low velocity scanning or reading beam maintains the exposed surface of the insulating coating at a fixed potential different from the potential established on the back plate of the target. The high velocity electrons from the photocathode penetrate the target and induce conductivity through the insulating material to establish on the scan surface of the target a charge pattern corresponding to the radiation pattern focused on the photocathode. The output signal derived from the target may be read out by the use of a low velocity scan beam such as used in the image orthicon in which the output signal is obtained from the electrons returned to a collector electrode. It is also possible to derive a signal from the conductive backplate in much the same manner as that utilized in the vidicon low velocity type scan operation. The electron bombardment induced conductivity type of target is important in many applications because of the amplification provided within the target itself. Although the sensitivity of the electron bombardment induced conductivity type of pickup tube is substantially superior to the conventional type of pickup tubes, it has several disadvantages.

Another type of pickup tube is described in U.S. Patent 3,189,781, issued June 15, 1965. The target described in U.S. Patent 3,189,718 operates on the principle of transmission secondary emission. Secondary electrons are emitted from the read surface of an insulative layer target in response to the write beam directed onto the opposite side of the target. The devices described above provide adequate sensitivity in most applications, but greater sensitivity is still in demand.

It is accordingly the general object of this invention to ice provide a new and improved electron discharge device incorporating a storage electrode which provides high amplification of the input signal.

It is another object to provide a new and improved target structure in which information impressed on an electron beam may be written on the target and then read out at a later time by an eletcron beam.

It is another object to provide a new and improved pickup tube in which an incoming light signal is converted into an electron image, directed onto an improved target structure providing amplification of the input signal and means for deriving the amplified signal from said target by an electron beam.

Briefly, the present invention accomplishes the above cited objects by providing a target structure which exhibits both the property of electron bombardment induced conductivity and transmission secondary emission. The storage target provides a storage means that exhibits transmission secondary emission effect in which secondary electrons are emitted from scan side to enhance the charging effect of the target surface due to the electron bombardment induced conductivity within the storage means.

Further objects and advantages of the invention will become apparent as the following description proceeds and features of novelty which characterize the invention will be pointed out in particularity in the claims annexed to and forming a part of this specification.

For a better understanding of the invention, reference may be had to the accompanying drawings, in which:

FIGURE 1 is an elevational view in section schematically representing a pickup tube in accordance with the teachings of this invention;

FIG. 2 is an enlarged elevational view in section, illustarting the electrode assembly in FIG. 1;

FIG. 3 is an elevational view in section of a modified electrode assembly that may be embodied in the tube shown in FIG. 1; and

FIG. 4 is an elevational view in section of a modified electrode assembly that may be embodied in the tube of FIG. 1.

Referring in detail to FIG. 1, there is illustrated a pickup tube comprising a glass envelope 10. At one end of the envelope .10, a light transmissive face plate 12 of a suitable material is provided, such as glass, having a coating 14 of a photoemissive material such as cesium anti-mony or a suitable multi-alkali photocathode provided on the inner surface thereof. The photocathode 14 is of any suitable material for the radiation scene to be viewed. It is also obvious that the photocathode 14 may be replaced with a conventional electron gun with associated deflection and the video signal applied to the electron gun. An electron gun 20 is provided at the opposite end of the envelope 10 for generating an electron beam which is directed onto a target member 30. The target member 30 is positioned between the electron gun 20 and the photocathode 14. Between the target member 30 and the photocathode 14 are provided a plurality of electrodes illustrated as 16 and 18 with suitable potentials provided thereto for accelerating and focusing the photoelectrons emitted from the photocathode 14 onto the target member 30.

The target member 30 as shown in FIG. 2 is comprised of a support ring 32 of a suitable material such as nickel having a suitable insulating support film 34 such as aluminum oxide attached to the metal ring 32. A conductive coating 35 is provided on the surface of the aluminum oxide supporting film facing the writing electron gun 20. A coating 36 is deposited upon the conductive layer 35 of a suitable high resistive material such as an insulative or semiconductive material which exhibits high elec- 3 tron bombardment induced conductivity. A suitable material for the layer 36 is arsenic trisulfide. Other suitable materials would be As Se ,.AsSe S, Sb S The layer 36 may be deposited in a vacuum to provide a bulk density deposit or in an inert atmosphere to provide a porous layer.

A layer 38 is deposited on the layer 36 and is of a material which exhibits both the property of transmission secondary emission and electron bombardment induced conductivity. A suitable material for layer 38 would be potassium chloride and again it may be a bulk density or a porous deposit. The layer 38 may also be any other suitable alkali halide KBr, KI or NaI or such materials as MgF SiO or MgO.

A conductive screen or mesh member 40 of a material such as nickel is provided adjacent the layer 38 and between the target member 30 and the scanning electrode gun 20. The grid 40 serves as a collector for secondary electrons emitted from the target member 30. The screen 40 with a potential of 300 volts contributes to maintaining a uni-form electric field between the grid 40 and the target 30. In addition, a conductive coating 44 is provided on the inner wall of the envelope in the space between the electron gun 20 and the target 30 for providing a suitable electrostatic field.

The electron gun 20 is of any suitable type for producing a low velocity pencil like electron beam to be scanned over the surface of the layer 38. The electron gun may consist of a cathode 22, cont-r01 grid 24 and accelerating grid 26. The gun electrodes 22, 24 and 26 along with the coating 44 provide a focused electron beam which is directed onto the target 30. Deflection means illustrated as a coil 50 is provided around the electron gun 20 for deflection of the electron beam to produce line and frame scansion over the surface of the target 30 in a conventional manner. A magnetic coil 52 is provided around envelope 10 to provide additional focusing of the electrons onto the target 30 from the photocathode 14 as well as providing focusing of the electron beam from the electron gun 20 onto the target 30.

A specific example of a suitable storage electrode 30 in accordance with the present invention and a method of forming such a structure will now be described. A thin sheet of aluminum foil approximately 99.7% pure and .0007 inch in thickness may be utilized. The sheet or foil of the desired dimensions, which may be of the order of two to three inches in diameter, is checked to insure that there are no pinholes therein. The foil is pressed and cleaned in an aqueous solution of ammonium cit-rate 3% by weight and anodized to the desired thickness of aluminum oxide by the adjustment of voltage. A piece of lead may be utilized for the cathode in this operation. By this procedure, an aluminum oxide coating of the desired thickness is formed on both surfaces of the aluminum sheet simultaneously. The anodized aluminum foil is removed from the electrolyte and washed in distilled water and then pure acetone. Next the anodized layer on one side of the aluminum is removed by treatment with a suitable caustic reagent, such as sodium hydroxide. After the sodium hydroxide has had an opportunity to act on the aluminum oxide film, the aluminum may be washed in distilled water and the aluminum oxide film on one surface is removed. The resulting structure provides an aluminum oxide support film 34 of three to five hundred angstrom units in thickness and a conductive layer 35 of aluminum of about five hundred to one thousand angstroms in thickness. The layer 36 may then be deposited upon the aluminum conductive layer by depositing a suitable material such as arsenic trisulfide by evaporation within a vacuum. The deposit that is obtained is of a density that is equal to the normally listed bulk density of the material. The thickness of the layer 36 is from a few hundred angstrom units to 1 to 2 microns depending on the voltages applied. It is also possible to evaporate the arsenic trisulfide in a few millimeters of mercury pressure of an inert gas such as argon so as to deposit a smoke like or porous layer whose average density is much less than that of the bulk material. The porous type structure is normally resorted to in an effort to obtain a low capacitance type target by obtaining increased thickness while still allowing the electrons to penetrate the layer.

The layer 38 may then be deposited upon the layer 36. A suitable material for the layer 38 is potassium chloride. Here again the layer may be of a porous or spongy deposit or a normal bulk type deposit. In the case of the porous deposit, the member is placed in an atmosphere approximately one millimeter mercury of argon gas. A quantity of about 16 milligrams of potassium chloride is placed in a suitable boat of a material such as tantalum. Material within the boat is placed at a distance approximately 3 inches from the layer 36. The potassium chloride is then heated to evaporate to completion a coating on layer 36 of the evaporated potassium chloride and of a density of approximately 87 micrograms per square centimeter. Such a layer has a thickness of approximately 20 microns.

In the operation of the device as illustrated in FIGS. 1 and 2, an input radiation scene is directed onto the photocathode 14 to provide photoelectron emission at a rate corresponding to the brightness of each of the elements of the radiation image directed thereon. The photoelectrons are accelerated to a velocity of about 7,000 to 25,000 volts depending on the thickness of the target 30 and may be focused to a reduced size upon the target electrode 30. The electrons will penetrate through the supporting layer of aluminum oxide 34, the conductive layer 35, the electron bombardment induced conductivity layer 36 and substantially through the transmission secondary emission layer 38. A field is established across the layer 36 and 38 by applying a potential of 10 to volts positive to the conductive layer 35 with respect to the cathode 22 of the electron gun 20.

The electron gun 20 in scanning the surface of layer 38 tends to maintain this surface at cathode or equilibrium potential. This writing action of the photoelectron produces charge carriers in the layer 36 and also in the layer 38 causing the elements to become conductive by the known electron bombardment induced conduction effect and in addition causes electrons to escape from the surface of the layer 36. The net result of the combined writing process is that the negative charge carriers excited by the electron bombardment induced conductivity action go to the positive target backplate 35 which is held at a potential of about 50 volts while holes migrate to the surface of the target. This tends to charge the surface of layer 38 toward the potential of the backplate 35. The secondary electrons emitted from the layer 38 are accelerated from the target surface and are collected by the collector mesh 40 which is at a potential of 300 volts position with respect to ground. This action also tends to charge the surface of layer 38 positive. All of these actions or mechanisms cooperate to increase the writing action and hence all tend to charge the surface positive with respect to the equilibrium potential established by the scanning electron gun. In the absence of signal, the action of the low velocity scan beam will bring the potential of the surface of the target or layer 36 to its equilibrium potential which is near cathode potential. The cathode 22 is connected to ground in the specific embodiment.

While it may be desirable under certain conditions as has been indicated in FIG. 1 and FIG. 2 to have the layers 36 and 38 consist of different materials, it is also possible to use an appropriate thickness of a single material or a mixture of materials. This is illustrated in FIG. 3 where a layer 39 has been substituted for the layers 36 and 38 illustrated in FIG. 2. In FIG. 3, the

layer 35 is utilized as the support and layer 34 used in FIG. 2 is not required. It has been found that many of the alkali halides have reasonably high secondary emission properties. In addition, it has been found that potassium iodide, sodium iodide, potassium chloride and cesium iodide also have appreciable electron bombardment induced conductivity gains. Potassium chloride in particular has been proved to be an excellent transmission secondary emission material. The thickness of one of these materials should be about 100 lLg./C1Tl. depending on voltages to be employed. The layer 39 provides the combined functions of the layers 36 and 38 in FIG. 2. The operation is the same as previously described with respect to FIGS. 1 and 2. It is desirable to optimize the substrate for optimum electron bombardment induced conductivity activity and the surface for optimum transmission secondary electron emission.

In FIG. 4, there is illustrated another modified structure which consists of three layers. Layer 41 is of a material which exhibits electron bombardment induced conductivity and emission of radiation such as light in response to electron bombardment of a material such as KI. Layer 43 is of a material that exhibits the property of electron bombardment induced conductivity and photoconductivity such as AS252. Layer 38 is a material that exhibits the properties of electron bombardment induced conductivity and transmission secondary emission and of such a material as KC]. It is also important, of course, to select materials for the layers 41 and 43 so as to have a reasonable spectral match.

In placing this target in the tube shown in FIG. 1, the operation of the tube is substantially the same. The writing beam in striking the target penetrates layer 41 and induces conductivity due to electron bombardment induced conductivity. In addition the light generated in layer 41 excites conductivity in layer 43 due to the photoconductive eifect. The writing electron beam also pene trates layer 4-3 inducing conductivity due to electron bombardment induced conductivity. The writing beam enters layer 38 and induces conductivity due to electron bombardment induced conductivity as well as secondary emission from the surface. All these effects tend to cause the surface of layer 33 to charge toward the positive potential of backplate 35. It is also understood that the materials need not be in distinct layers but can be intermixed to provide the enhanced effects. The materials could be evaporated simultaneously.

While there have been shown and described what are presently considered to be the preferred embodiments of the invention, modifications thereto will readily occur to those skilled in the art. It is not desired, therefore, that the invention be limited to the specific arrangements shown and described and it is intended to cover in the appended claims all such modifications as fall within the true spirit and scope of the invention.

I claim as my invention:

1. An electron discharge device comprising a target electrode including an electrically conductive layer, a first layer of material deposited on one surface of said conductive layer and having the property of electron bombardment induced conductivity and cathodoluminescence, a second layer of material deposited on said first layer and exhibiting the property of electron bombardment induced conductivity and photoconductivity, said second layer responsive to the light emission from said first layer, a third layer of material exhibiting the property of electron bombardment induced conductivity and transmission secondary emission wherein electrons entering one surface generate secondary electrons from the opposite surface, means for directing a writing electron beam having electrons of predetermined energy at said target to penetrate said conductive layer said first, second and third layers to establish a positive charge pattern corresponding to the energy of said writing beam from an equilibrium charge, means for establishing a field across said first, second and third layers to establish a conduction therein due to electron bombardment induced conductivity of said first, second and third layers and due to photoconductivity in said second layer and means for collecting the secondary electrons emitted from said third layer for charging the exposed surface in a positive direction and means for directing electrons at said exposed surface of said third layer to restore said third film surface to said equilibrium charge.

2. An electron discharge device comprising a target electrode including an electrically conductive layer, a first layer of a mixture of materials deposited on one surface of said conductive layer and having the property of electron bombardment induced conductivity, cathodoluminescence and photoconductivity, a second layer of material exhibiting the property of electron bombardment induced conductivity and transmission secondary emission wherein electrons entering one surface generate secondary electrons from the opposite surface, means for directing a writing electron beam having electrons of predetermined energy at said target to penetrate said conductive layer, said first layer and said second layer to establish a positive charge pattern corresponding to the energy of said writing beam from an equilibrium charge on the surface of said second layer, means for establishing a field across said first and second layers to establish a conduction therein due to electron bombardment induced conductively of said first and second layers and due to photoconductivity in said first layer and means for collecting the secondary electrons emitted from said second layer for charging the exposed surface of said second layer in a positive direction and means for directing electrons at the exposed surface of said second layer to restore the exposed surface to said equilibrium charge.

References Cited by the Examiner UNITED STATES PATENTS 2,527,981 10/ 1950 Bramley. 2,699,512 1/1955 Sheldon 313-651 2,739,258 3/1956 Sheldon 3 l 511 2,909,703 10/ 1959 Williams 31510 2,960,617 11/1960 Lodge et a1. 315-10 FOREIGN PATENTS 674,678 6/ 1952 Great Britain.

DAVID G. REDINBAUGH, Primary Examiner. 

1. AN ELECTRON DISCHARGE DEVICE COMPRISING A TARGET ELECTRODE INCLUDING AN ELECTRICALLY CONDUCTIVE LAYER, A FIRST LAYER OF MATERIAL DEPOSITED ONE SURFACE OF SAID CONDUCTIVE LAYER AND HAVING THE PROPERTY OF ELECTRON BOMBARDMENT INDUCED CONDUCTIVITY AND CATHODOLUMINESCENCE, A SECOND LAYER OF MATERIAL DEPOSITED ON SAID FIRST LAYER AND EXHIBITING THE PROPERTY OF ELECTRON BOMBARDMENT INDUCED CONDUCTIVITY AND PHOTOCONDUCTIVITY, SAID SECOND LAYER RESPONSIVE TO THE LIGHT EMISSION FROM SAID FIRST LAYER, A THIRD LAYER OF MATERIAL EXHIBITING THE PROPERTY OF ELECTRON BOMBARDMENT INDUCED CONDUCTIVITY AND TRANSMISSION SECONDARY EMISSION WHEREIN ELECTRONS ENTERING ONE SURFACE GENERATE SECONDARY ELECTRONS FROM THE OPPOSITE SURFACE, MEANS FOR DIRECTING A WRITING ELECTRON BEAM HAVING ELECTRONS OF PREDETERMINED ENERGY AT SAID TARGET TO PENETRATE SAID CONDUCTIVE LAYER SAID FIRST, SECOND AND THIRD LAYERS TO ESTABLISH A POSITIVE CHARGE PATTERN CORRESPONDING TO THE ENERGY OF SAID WRITING BEAM FROM AN EQUILIBRIUM CHARGE, MEANS FOR ESTABLISHING A FIELD ACROSS SAID FIRST, SECOND AND THIRD LAYERS TO ESTABLISH A CONDUCTION THEREIN DUE TO ELECTRON BOMBARDMENT INDUCED CONDUCTIVITY OF SAID FIRST, SECOND AND THIRD LAYERS AND DUE TO PHOTOCONDUCTIVITY IN SAID SECOND LAYER AND MEANS FOR COLLECTING THE SECONDARY ELECTRONS EMITTED FROM SAID THIRD LAYER FOR CHARGING THE EXPOSED SURFACE IN A POSITIVE DIRECTION AND MEANS FOR DIRECTING ELECTRONS AT SAID EXPOSED SURFACE OF SAID THIRD LAYER TO RESTORE SAID THIRD FILM SURFACE TO SAID EQUILBRIUM CHARGE. 